Today (Tues 2/24)

• Newspaper Articles: Ciara Rodwell and Melissa Regan

• Intro to Ozone, Ozone Hole, & Chapter 2• Light, matter, and human health!• Laboratory: Experiment 4 You design

investigation• Start Ozone Layer: Chemistry,

Destruction, Weather, and Policy

Chapter 2 – Protecting the Ozone Layer

The Ozone Hole

Learning Objectives• Ozone depletion and the ozone hole

– What is it, what is the status– What causes it?

• Basic Chemistry– Wavelength and frequency– Atomic structure– Molecules and models

• The ozone layer– Effects of UV radiation– Formation and destruction– CFC and ozone – reactions– International Policy

Good vs Bad Ozone

• GOOD• Stratrospheric Ozone• Absorbs dangerous

UV sunlight• Being depleted• 25-30 km altitude

• BAD• Tropospheric Ozone• Photochemical

Oxidant• Smog

Light, Matter, and Human Health

2.4

The Nature of Light

Low E

High E

Wavelength (λ) = distance traveled between successive peaks (nm).

Frequency (ν) = number of waves passing a fixed point in one second

(waves/s or 1/s or s-1 or Hz).

2.5

The energy of a photon of electromagnetic radiation is calculated by: E = h ν

where h = 6.63 x 10-34 J.s (Planck’s constant)

The wavelength and frequency of electromagnetic radiation are related by: c = λν

where c = 3 x 108 m/s (the speed of light)

Energy and frequency are directly related- higher frequency means higher energy.

Wavelength and frequency are indirectly related- higher frequency means lower wavelength.

2.5

What is the energy associated with a photon of light with a wavelength of 240 nm?

c = λν

= cνλ

ν = 3 x 108 m/s

E = h ν

240 nm x nm

10-9 m = 1.3 x 1015 s-1

E = (6.63 x 10-34 J.s) (1.3 x 1015 s-1)

E = 8.6 x 10-19 J

The Electromagnetic Spectrum: Wavelength

2.4

The various types of radiation seem different to our senses, yet they differ only in their respective λ and ν.

meters

1 m = 109 nm 1 nm = 10-9 m

Visible: λ

= 700 - 400 nm

R O Y G B I V

Infrared (IR) : longest of the visible spectrum, heat ray absorptions cause molecules to bend and stretch.

Microwaves: cause molecules to rotate.

Short λ range: includes UV (ultraviolet), X-rays, and gamma rays.

Decreasing wavelength

2.4

Energy (J)10-11 10-30

The Electromagnetic Spectrum 2: Wavelength, Energy Frequency

UV radiation has sufficient energy to cause molecular bonds to break

2.6

Fig. 2.7

Energy Distribution above the Earth’s atmosphereLink to Figures Alive

Oxygen and Ozone absorbance

O3O2

100 nm 200 nm 300 nm 400 nm 800 nm

Energy + 3 O2 2 O3

Ozone is an allotropic form of oxygen.

Energy must be absorbed (endothermic) for this reaction to occur.

Element Allotropesoxygen O2 , O3carbon graphite, diamond, buckminister fullerenes

2.1

An allotrope is two or more forms of the same element that differ in their chemicalstructure and therefore their properties.

Difference between O2 and O3 ? Allotropes

2.7

Biological Effects of Ultraviolet Radiation

The consequences depend primarily on:

1. The energy associated with the radiation.

2. The length of time of the exposure.

3. The sensitivity of the organism to that radiation.

An Australian product uses “smart bottle” technology; bottle color changes from white to blue when exposed to UV light.

The most deadly form of skin cancer, melanoma, is linked with the intensity of UV radiation and the latitude at which you live.

Skin Cancer Rates

Fig. 2.12

Detecting and Analyzing Light

• Spectrometer

• Bead?

Design your investigation…

• Questions: Where is UV and what materials are good at blocking it? How do the materials rank?

• Things to consider:– Quantification – Controls– Multiple Trials – ?

For Thurs (2/26)• Finish Reading Chapter 2• Start Homework #2 (Due 3/3)

Ozone Layer: Chemistry, Destruction, Weather, and Policy

Location of ozone layer.

Units for Ozone Measurement

• Dobson Units• 1 Dobson = 1 part per billion of ozone in

air.• In the 1960’s ozone minima in Antarctica

was about 300 Dobsons. In 30 years it has dropped to about 100 Dobsons.

Ozone depletion in Antarctica

How is Ozone Formed• The Chapman Cycle• Oxygen plus UV photon ozone• Step 1: O2 --------> O + O• Step 2: O2 + O ---------> O3• The sunlight (UV) excites the oxygen

molecules causing them to split into atomic oxygen. The released oxygen atom then combines with another oxygen molecule to form ozone.

Natural Ozone Destruction

• Chapman Cycle (cont’d)• Step 3:• O3 + UV photon --------> O2 + O• Step 4: O3 + O ---------> 2O2

Chapman Cycle

How is Ozone Destroyed… “Unnaturally”

• Mario Molina and F. Sherwood Rowland 1974 - first proposed ozone depletion by CFCs

• CFCs…..Chloro-Fluoro-Carbons.– Spray cans, air conditioners, refrigerators,

foams, fire extinguishers etc.• Very controversial originally• Won the Nobel Prize 1995 for this work• Banned in industrialized countries, still used

in developing countries

CFCs and ozone destruction

• Very stable - atmospheric residence times in the troposphere of 10 to over 100 years

• Eventually are mixed above the ozone in the stratosphere

• UVC breaks down CFCs to Cl free radicals (very reactive)

• Cl radicals destroy the ozone

Today (Thurs 2/26)• Finish Ozone Layer: Chemistry,

Destruction, Weather, and Policy• Focus Group Activity• Atoms, Covalent Bonds, and Lewis

structures• In-Class Worksheet #2

First, UV radiation breaks a carbon-halogen bond:

Photon (λ

< 220 nm) + CCl2 F2 .CClF2 + Cl. (free radicals)

2.9

How CFCs Interact with Ozone

2.9

2Cl. + 2O3 2ClO. + 2O2

2 ClO. ClOOCl

The chlorine radical attacks an O3 molecule:

Then two chlorine monoxide radicals combine:

UV photon + ClOOCl ClOO. + Cl.

ClOO. Cl. + O2

The ClOOCl molecule then decomposes:

The net reaction is: 2 O3 3O2

The Cl. radicals are free to attack more O3

The Cl. radicals are both consumed and generated; they act as catalysts

2.9

Experimental analyses show that as ClO. concentrations increase, ozone concentration decreases.

Fig. 2.16 Arctic vs. Antarctic

Polar Stratospheric

Clouds

Under extreme cold, with no wind, PSCs can form.

On the surface of the PSC, “safe” molecules are converted to reactive species.

Reactive species react with O3 .

Some interesting NASA links

• NASA video on ozone hole: http://learners.gsfc.nasa.gov/mediaviewer/ozStr/

• About the NASA spacecraft for measurements: http://aura.gsfc.nasa.gov/spacecraft/index.html

• NASA’s site on air pollution and Beijing Olympicshttp://aura.gsfc.nasa.gov/science/china.html

International Policy

• Montreal Protocol– Int’l agreement signed in 1987– More stringent in 1990, 1992, and 1997

• Black Markets• Substitutes – replace one or more of the

halons with hydrogen– Less stable

Impact of Policy

2.9

HCFCs are alternatives to CFCs: they decompose more readily in the troposphere so they will not accumulate to the same extent in the stratosphere.

Summary: Why is ozone depletion a problem?

• Increase UV radiation reaching the Earth’s surface– Increased skin cancers– Impact plant growth

• What is UV radiation?

Focus Group Activity

– Pair up as assigned– Read partner’s paragraphs– Discuss assignment with partner

• How did you think about pro-environment and smokestack sides?

• What challenges did you face when thinking about topic and this assignment? (Try to brainstorm solutions with your partners)

• Do your topics overlap? If so, how will you limit the overlap in your papers and presentations?

Atoms, Covalent Bonds, and Lewis Structures

Objectives• Protons, neutrons, and electrons• Atomic number = number of protons or electrons

in an electrically neutral atom • Mass number – What is it? What does it tell you?• Outer electrons – What are they?; Why are they

important; How do you use the periodic table to tell how many outershell electrons

• Isotopes – two or more forms of an element that differ in number of neutrons

• Draw Lewis structures of atoms• Draw Lewis structures of molecules

O8

16.00

Atomic number (Z)

Mass number (A)

-The number of protons andelectrons in a neutral atom.

-The sum of the protons and neutrons.

2.22.2

(nuclear charge)

2.2

Isotopes are two or more forms of the same element (same number of protons) whose atoms differ in number of neutrons, and hence in mass.

Isotopes of carbon: C-12, C-13, C-14also written as: 12C 13C 14C

The electrons in the outermost energy levels are called valence electrons.

2.2

The group number (of the representative elements) on the periodic table tells you the number of valence electrons.

1A

2A 3A 4A 5A 6A 7A

8A

Group 1A: 1 valence electron

Group 3A: 3 valence electrons

Electronic arrangements

Lewis structures of atomsWe represent atoms and valence electrons with Lewis structures…

N

O

B

5 valence electrons

6 valence electrons

3 valence electrons

N··· ··O··· ···B· ··

What about C and Cl?

2.3

Representing molecules with Lewis structures:

Typical valence for selected atoms = the # of bonds an atom typically forms

Element Typical valence

Classification

H, X (X= F, Cl, Br, I)

1 monovalent

O 2 divalent

N 3 trivalent

C 4 tetravalent

Molecular Structure• Covalent bond – a chemical bond where

electrons are shared by atoms

• Octet rule – general rule that in a molecule every atom is associated with 8 outer electrons (except H, which is associated with __ outer electrons)– Lewis structures for molecules – NH3 , – H2 , Cl2 , O2 , N2– Single bond, double bond, triple bond– examples

Representing molecules with Lewis structures:

2.3

Consider water, H2 O:

1. Find sum of valence electrons: 1 O atom x 6 valence electrons per atom = 6

+ 2 H atoms x 1 valence electron per atom = +28 valence electrons

2. Arrange the electrons in pairs; use whatever electron pairs needed to connect the atoms, then distribute the remaining electron pairs so that the octet rule is satisfied:

OH H

lone pair

bonding pair

OH H

2.3

Try these; draw valid Lewis structures for:

CO2

H2 S

NH3

HNO3

C OO

SH H

Sulfur is under oxygen; think of H2 O

N

O

O OH Can you draw other valid Lewis

structures for HNO3 ?

N

H H H

2.3

Representing molecules with Lewis structures:

Multiple bonds

O O H C C H

Triple bondDouble bond

Resonance StructuresOccasionally a single Lewis structure does not adequately represent the true structure of a molecule, so we use resonance forms:

N

O

O ON

O

O ON

O

O O

What about ozone, O3 ?

For Tues (3/3)• Homework 2 (Chapter 2) due• Read lab handout• Start reading Chapter 3• Start Homework 3 (due 3/17)

For Next Week’s Lab• Protection from UV light paper due